This invention relates generally to overvoltage input protection for voltage measurement instruments, and in particular to hand-held digital multimeters having transient overvoltage input protection.
A number of international standards have been implemented to govern safe use of electronic test equipment. To sell products in many markets, the products must conform to the standards, that is, the products must be in compliance with the essential requirements of such standards.
One such international safety standard is the International Electrotechnical Commission pilot standard (IEC) 644-1 (1992) entitled "Insulation Co-ordination For Equipment Within Low-Voltage Systems." Derived from IEC 644-1 is the product standard, IEC-1010, which uses the concept of overvoltage categories (CAT I-CAT IV) in setting forth the requirements of protection from overvoltage transients that may appear at the inputs of electronic measurement apparatus while a voltage source is being measured. Overvoltage transients usually are randomly-occurring, abnormally high voltage spikes or pulses that may last from a few nanoseconds in duration to a few milliseconds. Those contemplated by IEC-1010 are modeled by a pulse having a risetime of 1.2 microseconds, with a decay time, to 50% of peak, of 50 microseconds, and are designated as 1.2.times.50 .mu.sec pulses. Such overvoltage transients may be introduced from lightning strikes, high-voltage switching circuits effecting the source, ac power faults, or electrostatic discharge.
Overvoltage categories are used to differentiate locations based upon the probability that an overvoltage transient can cause a hazard. CAT IV is the utility feed, CAT III is the main distribution system, CAT ii is the plugs in the wall, and CAT I is controlled energy circuits. CAT III circuits represent the most hazardous since common test equipment is used in these circuits. CAT III circuits can provide substantial amounts of energy. If the measurement apparatus is a hand-held digital multimeter (DMM) designed for measuring voltages and other electrical phenomena, the result of an arc occurring inside the DMM with follow-through energy added can be catastrophic and/or injurious. Thus, the risk associated with CAT III circuits can be great.
The IEC-1010 standard provides for spacing requirements between conductive surfaces for a given maximum input voltage rating, pollution degree, and overvoltage category. These spacings are referred to as "creepage" distances along surfaces and "clearance" distances from point to point through air. Of course, larger distances enable electronic measurement instruments to withstand higher-rated voltages and some of the higher overvoltage transients that may be found on a system being measured.
Hand-held DMMs are typically small and compact, and voltage input terminals are necessarily physically located a short distance from one another. It would be desirable to provide a voltage measurement instrument having the highest transient overvoltage input protection attainable without adding complexity, bulk, or cost.